1. Field of the Invention
The present invention relates to a vaporizer of openrack type which is used to vaporize liquefied natural gas.
2. Description of the Prior Art
Among the known vaporizers for liquefied natural gas is the vaporizer of open-rack type as disclosed in Japanese Utility Model Laid-open No. 75388/1987.
An example of the vaporizer is shown in FIG. 12. It consists of a lower header 90 and an upper header 92. The lower header 90 extends in the direction perpendicular to the drawing and permits liquefied natural gas 91 to flow therein. The upper header 92 runs above and parallel to the lower header 90. The two headers 90 and 92 communicate with each other through a multiplicity of heat exchanger tubes 94. The heat exchanger tubes 94 are arranged in the axial direction of the lower header 90 (which is perpendicular to the drawing). Each of the heat exchanger tubes 94 is provided with a pair of fins (not shown) projecting in the axial direction of the lower header 90. Thus these heat exchanger tubes 94 form a heat transfer panel. Each of the heat exchanger tubes 94 is provided at its upper part with a seawater trough 96, so that seawater 98 (as a heating medium) flows along the surface of the heat exchanger tube 94. The heat of the seawater 98 vaporizes the liquefied natural gas, permitting it to rise through the heat exchanger tube 94. The vaporized natural gas is recovered through the upper header 92.
The above-mentioned apparatus poses a problem arising from the fact that the lower header 90 and the lower part of the heat exchanger tube 94 are in direct contact with liquefied natural gas and hence they are cooled to an extremely low temperature. The cooled surface frosts, forming an ice layer 99 (which functions as a heat insulating layer) as shown in FIG. 12 (right). The ice layer 99 prevents heat exchange, causing the lower part of the heat exchanger tube 94 to be kept at a still lower temperature. As the result, the temperature gradient in the heat exchanger tube 94 becomes similar to that in the liquefied natural gas flowing in it, as shown in FIG. 13.
Upon extreme cooling, the lower part of the heat exchanger tube 94 shrinks to such a great extent that even a slightest fluctuation in the distribution of seawater 98 changes the shrinkage of the individual heat exchanger tubes 94, which, in the worst case, causes local deformation of the heat exchanger tubes 94.
To cope with this situation, there was proposed an idea that each heat exchanger tube 94 is provided with a plurality of fins 94a so that it has an enlarged surface area for better heat transfer from seawater 98. A disadvantage of this idea is that the fins 94a make the surface configuration of the heat exchanger tube complex and the complex surface configuration easily becomes barnacled. The removal of barnacles is very difficult. In addition, the complex surface configuration is inconvenient for metal spraying with highly heat-conductive Al-Zn.
Any attempt to accelerate heat exchange between liquefied natural gas and seawater by increasing the heat transfer coefficient of the heat exchanger tube 94 itself is not successful because the ice layer 99 (as an insulating layer) becomes thicker in proportion to increase in the heat transfer coefficient. Consequently, it is impossible to prevent the lower part of the heat exchanger tube 94 from getting cooler. In addition, increasing the flow rate of both liquefied natural gas and seawater ends up with the thickening of the ice layer 99 because the increase in heat transfer coefficient by the increased flow rate of liquefied natural gas outweighs that by the increased flow rate of seawater.
The present invention was completed in view of the foregoing. Accordingly, it is an object of the present invention to provide a vaporizer for liquefied natural gas which prevents the heat exchanger tubes from being extremely cooled, without reducing the vaporizing efficiency.